1. Field of the Invention
The present invention relates to a belt for a continuously variable transmission which includes a metal ring assembly comprised of a plurality of endless metal rings laminated one upon another, and a plurality of metal elements each having a ring slot into which the metal ring assembly is fitted, and which is wound around a drive pulley and a driven pulley to transmit a driving force between the drive pulley and the driven pulley.
2. Description of the Related Art
There is such a belt for the continuously variable transmission, which is known from Japanese Patent Application Laid-open No. 2-225840, and in which the gravity center of the metal element is positioned in the vicinity of, or radially outside (above), a rocking edge in order to eliminate the gap formed between the adjacent metal elements in a chord section extending from the driven pulley to the drive pulley, and to bring the metal elements into engagement with the drive pulley in a correct attitude in which they are not inclined forwardly or rearwardly.
The conventionally known belt is intended to stabilize the attitude of the metal element in the chord section extending from the driven pulley to the drive pulley, but does not have an effect of stabilizing the attitude of the metal element which is in engagement with the pulley.
More specifically, if the metal element engaged with the pulley is pitched forwardly or rearwardly from a radial direction about the axis of rotation of the pulley, there is a possibility that the state of the metal element engaged with the pulley will become abnormal, whereby the attitude of the metal element will not only become unstable, but also front and rear edges of the saddle surface of the metal element will be brought into local contact with the lower surface of the metal ring assembly to exert an adverse influence upon the durability of the metal ring assembly. The direction and magnitude of the pitching of the metal element are determined depending on a tangent frictional force FV received from the surface of contact with the pulley by the metal element and an urging force E provided between the respective metal elements. The tendency of inclination of the metal element is particularly conspicuous in an exit region of the driven pulley. The reason will be described below.
It is known that the tangent frictional force FV received by the metal element 32 (see FIG. 3) from the drive pulley 6 or the driven pulley 11 is large in the exit region of the drive pulley 6 or the driven pulley 11, as shown in FIGS. 7A and 7B, and assumes a value about four times a value provided when the tangent frictional force FV has been averagely distributed over the entire wound region of the pulley 6 or 11 by a reason that the pulley 6 or 11 has been deformed, resulting in the concentration of an axial thrust, or for another reason. As is apparent from FIG. 3, the tangent frictional force FV is applied to the metal element 32 so as to fall the metal element forwardly in the direction of movement about the swinging center 44.
In addition, as shown in FIG. 7B, the urging force E provided between the metal elements 32 and inhibiting the inclination of the metal element 32 assumes a large value in the exit region (position b) of the drive pulley 6, but is 0 (zero) in the exit region (positioned) of the driven pulley 11. As is apparent from FIG. 3, a radial frictional force E1 is applied to the front and rear surfaces of the metal element 32 by the urging force E so as to fall the metal element 32 rearwardly in the direction of movement about the swinging center 44; namely, so as to oppose a moment generated by the tangent frictional force FV. Therefore, in a position where the tangent frictional force FV inclining the metal element 32 forwardly in the direction of movement is largest and the urging force E inhibiting the inclination of the metal elements 32 is 0 (zero), i.e., in the exit region (the position d) of the driven pulley 11, the metal element 32 is liable to be inclined to the largest extent.
The reason why the peak value of the tangent frictional force FV received by the metal element 32 from the driven pulley 11 reaches about four times the value provided when the tangent frictional force FV has been averagely distributed over the entire wound region of the pulley 11, is considered as follows:
FIG. 8 shows the results of the measurement of the tangent frictional force FV and the urging force E between the metal elements 32 to determine how they are varied in accordance with a variation in rotational angle xcex8. Points a, e, b, c, f and d on the axis of the abscissas correspond to the positions shown in FIG. 7B, respectively. As shown in FIGS. 9A and 9B, a sensor for measuring the urging force E between the metal elements 32 comprises an assembly which includes a beam formed into a U-shape and a strain gauge attached to an inner surface of the beam and which is mounted in a recess defined in the main surface of the metal element 32. The sensor measures the urging force E, based on the flexure of the beam produced by the urging force E. As shown in FIG. 10, a sensor for measuring the tangent frictional force FV of the metal element 32 comprises strain gauges attached in a pair of recesses defined in laterally opposite sides of an element body of the metal element 32, and measures the tangent frictional force FV based on the flexure of the element body produced by the tangent frictional force FV. It should be noted that the element body is flexed by an axial thrust transmitted from the V-face of the pulley to the metal element 32 and hence, an output from the strain gauge includes a component provided by the tangent frictional force FV and a component (a constant value) provided by the axial thrust transmitted from the V-face of the pulley.
As is apparent from FIGS. 7 and 8, the tangent frictional force FV assumes a peak value in the position in the exit region of the driven pulley 11, and the urging force E assumes the maximum value in the position f (point P) short of the position d. The urging force between the metal elements 32 is generated by the tangent frictional force FV received by the metal elements 32 from the pulley, and a rate of variation in urging force E is proportional to the tangent frictional force FV. Namely, an equation, dE/dxcex8=k * FV is established, wherein xcex8 represents a rotational angle of the pulley, and k is a constant.
In FIG. 8, when the urging force E assumes the maximum value at the point P, dE/dxcex8=0 is established and hence, the tangent frictional force FV is equal to 0 (zero) at the point P. As described above, the graph of the tangent frictional force FV in FIG. 8 includes the component provided by the tangent frictional force FV and the component provided by the axial thrust transmitted from the V-face of the pulley, but a substantial tangent frictional force FV resulting from the elimination of the component provided by the axial thrust transmitted from the V-face of the pulley can be detected by determining a point on the axis of the abscissas at which the tangent frictional force FV is equal to 0 (zero) from the point P at which dE/dxcex8 is equal to 0 (zero).
The tangent frictional force FV assuming 0 (zero) at the point f reaches a peak value AMAX at the point d corresponding to the exit of the driven pulley 11, but an integration value of tangent frictional force FV between the points f and d corresponds to one half of an area (AMAX * L1) of a triangle having a base provided by a distance L1 between the points f and d and a height provided by the peak value AMAX (see an obliquely-lined region shown in FIG. 8). On the other hand, the tangent frictional force FV is distributed uniformly in a region between the points c and d, which is a wound region of the driven pulley 11. If it is supposed that an average value of the tangent frictional force FV is AAVE, an integration value of the tangent frictional force FV between the points c and d corresponds to an area AAVE* L2 of a rectangle having a base provided by a distance L2 between the points c and d and a height provided by the average value AAVE. Thus, it can be seen that a value Amax/AAVE calculated according to (AMAX* L1)/2=AAVE* L2 is nearly equal to 4, and the peak value AMAX of the tangent frictional force FV is about four times the average value AAVE.
The metal element 32 receives the large tangent frictional force FV in the exit region of the driven pulley 11 where the urging force E between the metal elements 32 is equal to 0 (zero), as described above. For this reason, the pitching of the metal element 32 is produced by the tangent frictional force FV, thereby creating the above-described disadvantage.
The present invention has been accomplished with the above circumstances in view, and it is an object of the present invention to prevent the inclination of the metal elements engaged with the pulley to enhance the durability of the metal ring assembly.
To achieve the above object, according to one aspect of the present invention, there is provided a belt for a continuously variable transmission, comprising a metal ring assembly including a plurality of endless metal rings laminated one upon another, and a plurality of metal elements each having a ring slot into which the metal ring assembly is fitted, the belt being wound around a drive pulley and a driven pulley to transmit a driving force between the drive pulley and the driven pulley, the metal element including a saddle surface against which a lower surface of the metal belt assembly fitted in the ring slot abuts, pulley abutment surfaces provided below the ring slot to abut against the drive pulley and the driven pulley, and a rocking edge about which the preceding and succeeding metal elements are pitched relative to each other, wherein the distance A between the rocking edge and the saddle surface of the metal element and the distance L between the frictional force application point of the rocking edge and the pulley abutment surface of the metal element are set to be substantially equal to each other.
With the above arrangement, the distance A between the rocking edge and the saddle surface of the metal element and the distance L between the rocking edge and the frictional force application point of the pulley abutment surface of the metal element are set to be substantially equal to each other. Therefore, the frictional force application point can be set to be close to the rocking edge to the utmost, thereby reducing the pitching moment generated about the rocking edge by a tangent frictional force applied to the frictional force application point. As a result, even if a large tangent frictional force is applied to the frictional force application point of the metal element, the pitching caused by the tangent frictional force can be suppressed to the minimum to stabilize the attitude of the metal element, and it is possible to prevent an upper surface of the metal ring assembly from being brought into contact with an upper edge of the ring slot to reduce the durability of the metal ring assembly.
According to another aspect of the present invention, in addition to the arrangement of the first aspect, if the distance between the saddle surface and the lower end of the pulley abutment surface of the metal element is represented by B, and the thickness of the metal element is represented by t, equations, B/2=A+L and t=1.5A, are established, and the distances A and B are set, so that a relation, 2xe2x89xa6B/Axe2x89xa65, is established.
With the above arrangement, the distance A between the rocking edge and the saddle surface and the distance B between the saddle surface and the lower end of the pulley abutment surface satisfy a relation, B/Axe2x89xa65. Therefore, even when the tangent frictional force reaches a peak value in an exit region of the driven pulley, a pitching moment generated by the tangent frictional force and acting to project the metal element forwardly in a direction of movement can be suppressed by a pitching moment generated in the opposite direction by a load in the radial direction inwardly transmitted from the metal ring assembly to the ring slot, thereby reliably preventing the falling of the metal element. In addition, the distances A and B satisfy a relation, 2xe2x89xa6B/A and hence, it is possible to prevent the metal element from falling rearwardly in the direction of movement in the exit region of the driven pulley, and to effectively reduce the gap between the metal elements spaced apart from each other in a chord section extending from the driven pulley to the drive pulley.